1. Field of the Invention
The invention relates to a heat-resistant wire and a heat-resistant cable and, in particular, to a heat-resistant wire and a heat-resistant wire cable used for railroad vehicles.
2. Description of the Related Art
Electric wires and cables used in an application requiring high reliability (electric wires and cables for, e.g., nuclear power plants or railroad vehicles) are required to have not just only insulating properties but also to have mechanical characteristics and long service life and to be safe against fire accidents.
For electric wires and cables for, e.g., nuclear power plants, the environmental test method and the flame-retardant test method are defined in The Institute of Electrical and Electronics Engineers (IEEE) Standard 383 and Technical Report of IEE Japan (II), No.139 of Institute of Electrical Engineers of Japan (Japan Domestic Version). Materials to be used and test methods, etc., for railroad vehicle wires and cables are also defined even though the specification is slightly different in each country or region in the world.
Comparing to electric wires and cables used for general purposes, railroad vehicle wires and cables especially need to have not only just high flame retardancy but also mechanical characteristics in a high temperature condition with an electric current flowing, water resistance in a long-term water-immersion environment and resistance to oil such as lubricant oil due to the use environment.
On the other hand, weight reduction and downsizing are important issues for railroad vehicles. Electric wires and cables used therein are no exception and are required to be lighter in weight and to have a smaller diameter so as to fit in a reduced wiring space along with downsizing of vehicles.
Methods of reducing diameter of electric wire are, e.g., a method in which heat resistance of covering material is improved by increasing the allowable current for electric wire to allow a conductor cross sectional area to be reduced, and a method in which thickness of covering material is reduced by improving mechanical characteristics and insulation resistance of the covering material.
Although reduction in the diameter of conductor and the thickness of covering material can be achieved by using a fluorine resin or an engineering plastic having excellent heat resistance and mechanical characteristics as a covering material, such covering materials are expensive and cause over-engineering in case of electric wires used in an application in which the current flowing therethrough is little. In addition, such materials are often highly crystalline and rigid resins, and cause fitting properties to be poor.
JP-A-2013-214487 has proposed a multilayer insulated wire provided with a conductor, an inner layer covering the conductor and an outer layer further covering the inner layer. The inner layer is formed of a resin material containing at least calcined clay added in an amount of 10 to 100 parts by weight per 100 parts by weight of base polymer consisting mainly of modified poly(2,6-dimethyl phenylene ether), and the outer layer is formed of a polyester resin composition containing 50 to 150 parts by weight of polyester block copolymer, 0.5 to 3 parts by weight of hydrolysis inhibitor and 10 to 30 parts by weight of magnesium hydroxide per 100 parts by weight of base polymer consisting mainly of a polyester resin.